1. Field of the Invention
The present invention relates to a viewfinder optical system, an optical apparatus using the viewfinder optical system and a method for an expanding observation.
2. Related Background Art
A viewfinder optical system for a single-lens reflex (SLR) camera has been expected such that an observation magnification is high, a distance between the eyepiece and the eyepoint is long, and a diopter scale is adjustable. In order to make the observation magnification of a viewfinder high, the focal length of an eyepiece optical system is necessary to be shortened. However, the diopter scale of a viewfinder of an SLR camera is necessary to be set in the vicinity of −1 diopter, so that the substantial focal length of the viewfinder optical system is determined by the distance between the focusing screen and the eyepiece optical system. Accordingly, the simplest way is to shorten the optical path length of an erecting image forming member, and the distance between the focusing screen and the eyepiece optical system.
However, in this way, since the eyepiece optical system falls back from the rear side of the camera to the front side thereof, it becomes difficult to put an observer's eye near to the eyepiece optical system. On the other hand, when the distance between the eyepiece and the eyepoint is made sufficiently long, an effective diameter at the exit surface of the erecting image forming member has to be large resulting in inevitably lengthening the optical path length of the erecting image forming member, so that observation magnification of the viewfinder optical system becomes small. As described above, in a viewfinder optical system for an SLR camera, to heighten observation magnification and to lengthen the distance between the eyepiece and the eyepoint have been contrary to each other.
In the case of an SLR digital camera, since a display member for confirming an image is disposed on the rear side thereof, it becomes further difficult to observe the viewfinder of the camera unless the eyepiece optical system is disposed closer to the rear side of the camera than before.
Moreover, when the observer wears glasses, unless the diopter scale is fitted to the observer, the observer's eye has to be closer to the eyepiece optical system, so that the observer finds it difficult to observe the viewfinder of the camera. Therefore, a diopter adjustment mechanism becomes an indispensable function for an eyepiece optical system.
Other than the conventional method to shorten the optical path length of the erecting image forming member, there has been proposed a method to lengthen the total length of the eyepiece optical system with heightening observation magnification by means of increasing the number of lenses composing the eyepiece optical system (for example, Japanese Patent Application Laid-Open No. 2003-215471).
However, the disclosed example of Japanese Patent Application Laid-Open No. 2003-215471 cannot secure sufficient observation magnification. Accordingly, when a small-size solid-state imaging device, which is used in an SLR digital camera, is used, the image capable of being observed by the viewfinder optical system becomes extremely small. A viewfinder optical system for an SLR camera is expected such that observation magnification is high, a distance between the eyepiece and the eyepoint is long, and diopter is adjustable. When all the requirements are made to be satisfied, the whole optical system becomes complicated, so that it becomes difficult to satisfactorily show respective functions' ability.